Crosstalk mitigation

ABSTRACT

Methods and devices are provided for estimating crosstalk from a legacy line to a vectored line.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/629,506 filed on Feb. 24, 2015, the contents of which areincorporated by reference in their entirety.

FIELD

The present application relates to crosstalk mitigation, sometimes alsoreferred to as vectoring.

BACKGROUND

Far end crosstalk (FEXT) is a dominant source of disturbance intransmission systems where for example a plurality of communicationlines is located close to each other. Such a situation may occur whenthe plurality of communication lines is provided in a so-called cablebinder. To mitigate far end crosstalk, vectoring was developed.Vectoring is essentially a technique where signals transmitted via aplurality of communication lines are processed jointly, either beforebeing transmitted or after being transmitted. In the first case,vectoring is also referred to as crosstalk precompensation, and in thelatter case vectoring is also referred to as crosstalk cancellation.

For VDSL2 systems (very high bit rate digital subscriber line 2),vectoring was standardized in ITU-T recommendation G.993.5. According tothis standard, training sequences are transmitted by modulatingpredefined sequences onto so-called synchronization symbols, alsoreferred to as sync symbols. Sequences for different lines are selectedto be orthogonal to each other. By evaluating error values (differencesbetween received and transmitted sequences), crosstalk between lines canbe estimated. Based on this estimation, crosstalk may be mitigated. Forexample, for crosstalk precompensation data transmitted via the line ispre-distorted by the data of every other line weighed by the respectivecrosstalk transfer function. A similar weighting is performed aftersignals are received in case of crosstalk cancellation.

However, for this mechanism, in particular the estimation of crosstalkcoefficients, to be operational, devices involved (for example centraloffice equipment and customer premises equipment) have to implementvectoring capabilities, e.g. comply with the above-mentioned standard.However, legacy devices exist which may for example be VDSL2 equipmentnot adapted to vectoring, i.e. not adapted to implement the mechanismsas specified in G.993.5. In some cases, communication lines (alsoreferred to as legacy lines herein) coupled to such legacy devices arelocated close to other communication lines employing vectoring.Crosstalk from such legacy lines to vectored lines and vice versa cannotbe cancelled or mitigated following the above-mentioned standard.Therefore, in conventional solutions the full benefit of vectoring mayonly be obtained in cable binders or similar arrangement of lines whichall follow a common vectoring implementation, e.g. a vectoring standard.On the other hand, as legacy lines exist, it may be of interest to beable to take legacy lines into account when performing vectoring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system according to an embodiment.

FIG. 2 is a flow chart illustrating a method according to an embodiment.

FIG. 3 is a block diagram illustrating crosstalk in a system accordingto an embodiment.

FIG. 4 is a block diagram illustrating a system according to anembodiment.

FIG. 5 is a block diagram illustrating a system according to a furtherembodiment.

FIG. 6 is a flow chart illustrating a method according to an embodiment.

FIG. 7 is a diagram illustrating a superframe structure used in someembodiments.

FIG. 8 is an illustrative diagram illustrating a crosstalk estimationusing grid tones.

DETAILED DESCRIPTION

In the following, various embodiments will be described in detailreferring to the attached drawings. These embodiments serve as examplesonly and are not to be construed as limiting. For example, whileembodiments may be described as comprising a plurality of features orelements, in other embodiments some of these features or elements may beomitted and/or may be replaced by alternative features or elements.Also, further features or elements in addition to the ones explicitlyshown or described may be provided.

Features from different embodiments may be combined with each otherunless noted otherwise.

Any connection or coupling shown in the drawings or described herein maybe a wire-based connection or coupling or a wireless connection orcoupling unless noted otherwise. Furthermore, connections or couplingsmay be direct connections or couplings (i.e. connections or couplingswithout intervening elements) or indirect connections or couplings (i.e.connections or couplings with one or more additional interveningelements), as long as the basic purpose of the connection or coupling,for example to transmit a certain kind of information or to transmit acertain kind of signal, is essentially maintained.

Terminology used herein may have a meaning as defined in various xDSL(digital subscriber line) standards, for example ITU-T recommendationslike ITU-T G.993.5. xDSL is a generic term used herein to refer to anykind of DSL system like ADSL (asymmetric DSL), ADSL2, VDSL (very highbit rate DSL), VDSL2 or also the upcoming G.fast. However, applicationof the techniques disclosed herein is not necessarily limited to xDSL.Vectoring as used herein refers to a technique defined for example inG.993.5 which reduces far end crosstalk by joint processing of signalsto be sent via a plurality of communication lines or received via aplurality of communication lines. A vectored group refers to a group ofcommunication lines to which vectoring is applied. A joining line mayrefer to a line which is to join the vectored group. Such a situationmay for example occur when a line which previously was inactive becomesactive. A legacy line refers to a line coupled to at least onecommunication device not adapted to vectoring, e.g. not implementedaccording to a vectoring standard.

FIG. 1 illustrates a simple diagram of a communication system accordingto an embodiment.

The system of FIG. 1 comprises a central office device 10. The term“central office” as used herein does not necessarily imply that thecorresponding device has to be located at the office of the provider,but may relate to any equipment on a service provider's side and mayinclude for example DSLAMs, street cabinets or the like. Central officedevice 10 is coupled with a plurality of customer premises devices(CPEs; customer premises equipment) 11 to 13 via a plurality ofcommunication lines. While three customer premises devices 11 to 13 areillustrated in FIG. 1, this serves merely as an example, and any numberof customer premises devices may be present.

Central office device 10 may communicate with customer premises devices11 to 13 for example using xDSL communication. To this end, centraloffice device 10 may for example comprise a transceiver for each of thecommunication lines, and each of customer premises devices 11 to 13 mayalso comprise a transceiver. As the general structure of such xDSLtransceivers is known and at least in part defined in various xDSLstandards, it will not be discussed in more detail here.

As will be discussed later in greater detail, some of the transceiversmentioned above may be adapted to employ vectoring, for example asdefined in a standard like ITU-T G.993.5. For such transceivers, e.g.modified synchronization symbols (sync symbols) may be used for vectortraining. For example, orthogonal sequences of modified sync symbols maybe transmitted via a plurality of lines to determine crosstalk, forexample as defined in ITU-T G.993.5. Other transceivers may be legacytransceivers not explicitly adapted to vectoring. For example, one ormore customer premises devices 11 to 13 may be legacy devices. The term“legacy” as used herein may refer to devices, systems, entities etc. notbeing adapted to employ standardized vectoring. For such legacy devices,in embodiments, e.g. central office device 10 may be adapted todetermine crosstalk from one or more legacy lines to one or morenon-legacy lines, also referred to as vectored lines herein. Suchcrosstalk may e.g. be determined based on an analysis of random datatransmitted from legacy devices to the central office device or based onorthogonal sequences transmitted from the central office device to alegacy CPE device even if the legacy CPE device cannot returncorresponding error values (as it is a legacy device not adapted tocrosstalk estimation). Examples and more details will be describedlater.

A communication direction from central office device 10 to customerpremises devices 11 to 13 will be referred to as downstream directionherein, and a communication direction from customer premises devices 11to 13 to central office device 10 will referred to as upstreamdirection, as it is common in the art.

In FIG. 2, a flow chart illustrating a method according to an embodimentis shown. While the method is illustrated as a series of acts or events,the order in which these acts or events are described is not to beconstrued as limiting. In particular, the order in which the acts orevents occur may differ from the order shown, and acts or events mayalso be performed simultaneously, for example by different parts of asystem.

At 20, crosstalk related to a vectored set of lines of a communicationsystem is determined based on modified synchronization symbols. Thisestimation may for example be performed as defined in ITU-Trecommendation G.993.5. Crosstalk related to the vectored set of linesmay in particular be crosstalk from lines of the vectored set to otherlines of the vectored set or, in some embodiments, a non-legacy linethat just became active and is about to join the vectored set.

At 21, crosstalk from a legacy set of lines to the vectored set of linesis estimated, e.g. based on modified synchronization symbols in thedownstream direction or based on signals other than modifiedsynchronization symbols in upstream direction and/or downstreamdirection. Examples will be described later. The legacy set of lines mayfor example comprise one or more lines coupled to at least onecommunication device not adapted to perform vectoring as defined inITU-T G.993.5.

A set of lines, as used herein, may include one or more lines.

It should be noted that the estimation of 20 and the estimation of 21each may be split in time or may be performed simultaneously. Forexample, in some scenarios at initializing a communication system onlysome lines (each either of the vectored set or of the legacy set) may beactive, and crosstalk for these lines may be estimated at 20 and 21.Later on, further lines may become active, and their crosstalk may beestimated when they become active according to 20 or 21 of FIG. 2.

At 22, the crosstalk is reduced based on the estimates. For examplevectoring, i.e. joint processing of signals to be sent or signalsreceived, is adapted based on the estimates.

To illustrate further, more detailed explanations of systems accordingto embodiments will be explained with reference to FIGS. 3 to 5. Themethod of FIG. 2 or methods illustrated further below may for example beemployed in the systems of FIGS. 3 to 5 or also in the system of FIG. 1,but are not limited thereto.

FIG. 3 illustrates a communication system according to a furtherembodiment. On a central office side, for example in a DSLAM or streetcabinet, the system of FIG. 3 comprises a plurality of transceivers 30Ato 30C, collectively referred to as transceivers 30. While threetransceivers 30A to 30C are explicitly shown in FIG. 3, this is not tobe construed as limiting, and any number of transceivers may beprovided. Transceivers 30A to 30C may be xDSL transceivers. Transceivers30 may be collocated to be able to employ vectoring or non-collocated tobe able to employ vectoring over for example multiple DSLAMs.

Transceivers 30 communicate via respective communication lines 32A to32C (collectively referred to as communication lines 32) with customerpremises side transceivers 33A to 33C (collectively referred to astransceivers 33). Also here, the number of three communication lines 32and three transceivers 33 serves merely as an example.

Arrows 34 indicate far end crosstalk (FEXT) between lines 35, which farend crosstalk may be reduced or eliminated by vectoring. Lines 32 in theexample of FIG. 3 are arranged in a cable binder 31, which means thatthey are in comparatively close proximity to each other. This makes themprone to crosstalk like FEXT 34.

FIG. 4 illustrates a VDSL2 system according to some embodiments. WhileFIG. 4 and also FIG. 5 further below illustrate VDSL2 systems, this isnot to be construed as limiting, as techniques disclosed herein may alsobe applied to other xDSL systems including G.fast. The system of FIG. 4comprises a central office device 40, in the example shown a DSLAM.Central office device 40 comprises a plurality of VDSL2 (VDSL2 CO)transceivers 42 (for example n transceivers), which are coupled to avectoring device 41. Vectoring device 41 may provide vectoring thoughjoint processing of signals to be sent by transceivers 42 or of signalsreceived by transceivers 42. Transceivers 42 communicate with customerpremises side VDSL (VDSL2 CPE) transceivers 46, 47 via a plurality ofcommunication lines 48A to 48D (collectively referred to ascommunication lines 48). The number of VDSL transceivers 42, 46, 47 andthe number of communication lines 48 illustrated in FIG. 4 is merely anexample, and other numbers may be used as well.

Lines 48A to 48F are located in a first cable binder 43. Lines 48D to48F then terminate at VDSL2 CPE transceivers 46, which may be locatedcomparatively close to each other. Other lines, in the example shownlines 48A to 48C, continue through a second cable binder 44 to VDSL2 CPEtransceivers 47. This is merely an example scenario, and for examplefurther communication lines may be provided which terminate it furthertransceivers through further cable binders or also outside cablebinders.

Generally, crosstalk between lines in a common cable binder may behigher than crosstalk between lines not sharing a cable binder.Furthermore, crosstalk between lines which run in a common cable binderfor a longer distance (for example between lines 48A to 48C) tend to bestronger than between lines which share a cable binder only for ashorter distance (for example between lines 48D to 48F).

Each of transceivers 42, 46 and 47 may be a VDSL2 transceiver adapted toperform vectoring (for example in compliance with ITU-T G.993.5) or maybe a legacy transceiver not adapting to perform any kind of standardizedvectoring. This is further illustrated in FIG. 5.

FIG. 5 shows the communication system of FIG. 4, wherein some of thetransceivers are marked (vectored), indicating that they employvectoring, and some are indicated as (legacy), indicating that they arenot adapted to perform standardized vectoring. This may in particularapply to CPE receivers 46, 47. For example, of transceivers 47transceivers #1 to #i are “vectored”, while transceivers #i+1 to #k are“legacy”, and for transceivers 46 transceiver #k+1 to #m are vectored,and transceivers #m+1 to #n are legacy transceivers. The correspondingtransceivers 42 in central office device 40 may operate according to theCPE transceivers, i.e. operate either as vectored or legacytransceivers. It should be noted that in some embodiments on the centraloffice side, transceivers may be switched between a vectored mode and alegacy mode.

In the embodiments of FIGS. 3 to 5, for example a method as generallydiscussed with respect to FIG. 2 may be implemented. More detailedapproaches to including legacy lines and transceivers in a vectoredsystem will be discussed below.

The discussion will be made separately for downstream direction andupstream direction. Generally, in the downstream direction, a centraloffice device like the ones illustrated in FIGS. 1-5 controls thesignals to be sent to the CPE devices. Conversely, in the upstreamdirection, a central office device in many cases has only very limitedpossibilities to influence signals sent by the CPE devices. For example,in legacy VDSL2 systems the central office may only influence a transmitpower of CPE devices.

First, the downstream direction will be discussed.

In the downstream direction, a central office device as mentioned abovemay control signals to be sent over a legacy DSL connection.

In embodiments, on legacy lines, a suitable signal, referred to asdownstream (DS) vector probe signal in the following, is transmittedover one or more legacy lines in embodiments for example to estimate adownstream crosstalk transfer function from a legacy VDSL line to avectored VDSL line. Such a DS vector probe signal may in someembodiments be transmitted before starting VDSL2 training on the legacyline. In some embodiments, such a DS vector probe signal may correspondto a sequence of sync symbols modified by an orthogonal sequence(orthogonal to sequences used on other lines as for vectored lines, withthe difference that a legacy CPE device cannot return error values as itis not adapted to receive such a sequence and evaluate it). Again, whileVDSL2 is used as an example herein, techniques discussed herein may alsobe applicable to other communication types, for example other xDSLcommunications.

FIG. 6 shows an example embodiment of a method for a case where a legacyline is about to become active, thus potentially disturbing an alreadyactive vectored group of lines. On the left side of FIG. 6, acts orevents performed on a CPE side are illustrated, and on the right side ofFIG. 6 under the heading CO acts or events are performed on a CO sideare illustrated. Apart from the acts or events shown, further acts orevents may be performed, for example as laid down in appropriatestandards like xDSL standards, for example VDSL2 as standardized inITU-T G.993.2.

As already explained for the embodiment of FIG. 2, the order the acts orevents are illustrated in FIG. 6 is not to be construed as limiting.

Prior to the joining of the new line, at 60 a CPE device (for exampletransceiver) is idle, and at 65 a corresponding part of a CO device (forexample an associated CO transceiver like one of transceivers 42 of FIG.5) is idle. At 61, the CPE device transmits a joining request to the COdevice, indicating that the line wants to become active. At 66, the COdevice transmits a DS vector probe signal to the CPE device to be ableto estimate crosstalk from the joining line to lines already in thevectored group. The DS vector probe signal may in some embodiments be asignal corresponding to a crosstalk estimation sequence as defined inITU-T G.993.5 and may comprise e.g. modified sync symbols. The DS vectorprobe signals may be transmitted on symbols (e.g. sync symbols) on thelegacy line at the same point in time as synchronization symbols on thevectored (non legacy) lines. Therefore, in embodiments, a CO maytransmit a standard vector training sequence on legacy lines withoutreceiving error signals in return from the respective legacy CPE device.In other embodiments, random data or other signals may be used.Crosstalk is then estimated based on error values returned by vectoredCPE receivers (the DS probe signal on the legacy line influences thereceived sequences on vectored lines).

After crosstalk has been estimated, at 62 and 67 pre-training isperformed followed by training at 63 and 68 of FIG. 6. Before or inpre-training 62, 67, signals may be precompensated based on theestimated crosstalk from the joining line to vectored lines, such thatthe subsequent training of the joining line does not adversely affectthe vectored lines as regards crosstalk. Training may be performed asdefined in a respective standard for the joining legacy line, forexample according to legacy VDSL2 (without vectoring).

After the training at 63 and 68, at 64 and 69 showtime, i.e. regulardata transmission, begins.

It should be noted that while FIG. 6 illustrates the case of a joiningline, it may also be applied to two or more joining lines.

Next, suitable DS vector probe signals will be discussed in more detail.

Generally, most DSL systems employ discrete multitone modulationtechniques (DMT) where data is modulated onto a plurality of differentso-called tones, i.e. different carrier frequencies. In embodiments, togenerate the DS vector probe signals for a legacy line, a set of tonesfrom an overall available number of tones is selected for crosstalkestimation. The selected tones are also referred to as grid tones. Thesegrid tones, for example in sync symbols or at positions of sync symbolsin vectored lines, may then e.g. be modulated by an orthogonal sequence.

FIG. 8 illustrates an example where every 11^(th) tone is used as such agrid tone (in the example of FIG. 8 for example tones #1, #12, #23, #34etc.). However, this serves only as an example and is not to be regardedas limiting. These grid tones in embodiments are modulated with adedicated orthogonal sequence, which may be different from, inparticular orthogonal to, orthogonal sequences used for crosstalkestimation on the respective vectored lines, for example may be aspecific reserved sequence.

Instead of an equally spaced set of grid tones as illustrated in FIG. 8,other criteria may equally be used, for example an optimizationcriterium like bit loading. For example, only tones may be used ontowhich only a few bits (or only one bit) can be loaded, such that almostno loss of data transmission capacity occurs when using the grid tones.In yet other embodiments, instead of grid tones all tones may be used.

Crosstalk transfer functions for tones between the grid tone may then beestimated by interpolation, for example linear interpolation ornon-linear interpolation, between the crosstalk estimates obtained forthe grid tones.

In other embodiments, as an alternative or in addition to transmitting adedicated DS vector probe signal, data symbols, e.g. during showtime,may be used for crosstalk estimation. Such an approach may also be usedfor the upstream direction, as will be discussed later.

In some embodiments, measures may be taken to prevent the CPE from usingthe tone set (e.g. the grid tones discussed above) used for the DSvector probe signal for data transmission. Such measure may include:

In an embodiment, artificial noise may be added to all tones of the toneset (e.g. grid tones), for example during a channel estimation orsimilar part of initializing a line. Such artificial noise may be addedby transmitting corresponding noise signals (e.g. random signals) on therespective tones by the CO. By adding such noise, the CPE device maycome to the conclusion that no bits may be loaded on the respectivetones, such that the CPE device refrains from using them.

In an embodiment, the tones of the tone set (e.g. grid tones) may beexcluded from the supported set of tones as defined for example in ITU-TG.993.2 (defining VDSL2). In other embodiments, the tone set might beexcluded by means of setting a CARMASK as defined for example in G.997.1defining physical layer management for digital subscriber linetransceivers accordingly. CARMASK is a configuration parameter whichmasks specific subcarriers (tones). In situations other than DSLcommunication, other corresponding masking parameters may be used.

In an embodiment, a firmware update to a legacy CPE device may be usedto force the respective CPE devices not to use any tone of the selectedtone set.

In some embodiments, the transmit spectrum of the legacy line may beoptimized depending on the legacy CPE device's capability to follow themethods for reserving the tones explained above.

Therefore, in embodiments, an estimation of a crosstalk transferfunction from a legacy VDSL line to vectored VDSL2 lines in training orin showtime becomes possible and may be made based on an orthogonalsequence which is sent on grid tones in VDSL2 legacy symbols which aretransmitted at the same point in time as the sync symbols (e.g. syncsymbols, but not limited thereto) of the vectored VDSL lines.

In some embodiments, as mentioned above, sync symbols used also inlegacy VDSL may be modified with a specific data pattern or sequence forestimating the crosstalk transfer function as DS vector probe signal.The structure of a VDSL frame is shown in FIG. 7. 256 data symbols 70Ato 70D are shown followed by a sync symbol 70E. While in vectoredsystems following G.993.5 synchronization symbols for different linesare aligned in time, for legacy line such an alignment does notnecessarily exist. A DS vector probe signal as explained above, e.g. ina VDSL2 system, may be transmitted after a first handshake (for exampleas defined in ITU-T G.994.1) and before the training of a joining legacyline.

The downstream synchronization symbols of the legacy lines in someembodiments nevertheless may be aligned by the CO device tosynchronization symbols of vectored lines in the downstream direction.It should be noted that such an alignment may not be possible in theupstream direction, as here, as explained above, the CO device may haveless influence on the transmitted data.

The estimation of the downstream crosstalk transfer function from theVDSL legacy line into vectored lines may then be performed essentiallyin the same way as between the VDSL vectored lines, e.g. by evaluatingerror signals received from CPE devices coupled to the vectored lines.For example, in VDSL2 implmenting ITU-T G.993.5, CPE devices coupledvectored lines report error values, and based on these error values, thecrosstalk from the legacy line to the vectored line may be estimated.The error values may be indicative of a difference between transmittedcrosstalk estimation signals (e.g. orthogonal sequences) and receivedsignals. Crosstalk transfer functions thus estimated may be re-usedlater for a new training, for example when a legacy line becomesinactive and active again later.

Next, the upstream direction will be discussed.

In upstream direction the CO in some embodiments has only control overthe transmit power of the legacy VDSL2 signal but not over theinformation contained in signals being transmitted by the CPE. Thismeans transmission in upstream cannot be limited to sync symbols in anytraining state (as in legacy devices there is no restriction regardingsent signals during training, whereas in vectored systems untilcrosstalk is estimated often only sync symbols are sent on a joiningline to avoid disturbing data transmission on already vectored lines)but data which is impacting vectored VDSL2 may be transmitted in allsymbols on legacy lines instead. Thus the crosstalk impact e.g. of ajoining legacy VDSL2 line to vectored VDSL2 lines already in showtime insome embodiments may be limited by reducing the transmit power of thelegacy VDSL2 lines until the crosstalk coefficients have been adaptedand/or by increasing a margin of the vectored lines (e.g. loading lessbits than possible, using higher transmit power etc.) in showtime duringa crosstalk adaptation time.

In some embodiments, a significant reduction of the transmit power inupstream of a joining legacy VDSL2 line is achieved by forcing anappropriate upstream PSDMASK (as defined in VDSL2) to the appropriatelegacy CPE device (e.g. legacy CPE devices of FIG. 1, 4 or 5). Duringoperation with reduced transmit power, crosstalk from the legacy line tovectored lines may be determined, and vectoring may be adaptedcorrespondingly. In some embodiments, in such a case two trainings ofVDSL2 legacy lines may be implemented—for a first training a reducedupstream transmit power may be demanded from the VDSL2 legacy CPE by theVDSL2 legacy CO e.g. via a signal 0-SIGNATURE (e.g. as defined in legacyVDSL2) to minimize the crosstalk into the vectored lines until thecrosstalk coefficients are adapted whereas for the second training whichis being executed after crosstalk adaptation the full upstream transmitpower is applied, to train the lines for transmission with full transmitpower. The reduced upstream transmit power in embodiments may bedetermined such that none of the vectored lines drops its link due tothe joining legacy VDSL2 line under assumed worst case crosstalkconditions.

Alternatively or combined with the upstream transmit power reduction ofthe joining VDSL2 legacy line mentioned above, in some embodiments theVDSL2 vectored lines in showtime may be protected by reducing theirupstream bit loading when a legacy VDSL2 line joins and by this meansincrease the upstream margin of the VDSL2 vectored lines in showtime.For this scheme one training only is required for the legacy VDSL2 line.Reduction of upstream bit loading is achieved for example by forcing anupstream SRA (Seamless Rate Adaptation) or an SOS (Save Our Showtime),which may be implemented in any conventional manner by the CO.

Because no vectoring specific upstream training signal can be applied toVDSL2 legacy lines, the estimation of the upstream crosstalk transferfunctions from legacy VDSL2 to vectored VDSL2 lines in embodiments mayrely on symbols containing mutual statistical uncorrelated signalstransmitted on legacy VDSL2 lines in upstream direction. For example,data symbols transmitted in showtime are by nature statisticallyuncorrelated and so these data symbols in some embodiments are used forestimating the upstream crosstalk from legacy VDSL2 lines to vectoredVDSL2 lines. In other embodiments, additionally or alternatively theymay be used for estimation in the downstream direction. Furtheron thestatistics of the sync symbols being used for crosstalk estimationbetween VDSL2 vectored lines must not be disturbed by the crosstalk oflegacy VDSL2 symbols. Thus, in embodiments measures are taken such thatuncorrelated signals of the legacy VDSL2 line(s) (e.g. random data) arepresent at sync symbol position of the VDSL2 vectored lines which carryvectoring specific data (e.g. the aforementioned orthogonal sequences)being used for crosstalk estimation between VDSL2 vectored lines. Forexample, in embodiments a data symbol of VDSL2 legacy lines overlapswith sync symbols of the VDSL2 vectored lines (which e.g. according toITU-T G.993.5 are transmitted at the same time for all vectored lines).If by accident the sync symbol of a VDSL2 legacy line overlaps with thesync symbol of the VDSL2 vectored lines, this VDSL2 legacy line inembodiments is retrained with its upstream sync symbol being shiftedwith respect to the VDSL2 vectored sync symbols. Because VDSL2 trainingis controlled by the CO, the upstream sync symbol is for example shiftedby shifting the start of the retrain with respect to the upstream syncsymbol of the VDSL2 vectored lines compared to the previous training.While VDSL2 is used as an example above, the techniques disclosed abovemay also be applied to other kinds of communication, e.g. other xDSLcommunication techniques.

Therefore, in the upstream direction, as data symbols are used, care istaken that data symbols (e.g. random data) at least partially overlapwith the symbols on the vectored line.

Essentially, in some embodiments the random data at the position of thesync symbols for purposes of crosstalk estimation may be treated asanother orthogonal sequence by the CO device, for example by a vectoringentity of the CO device. While in contrast to orthogonal sequences usedfor crosstalk estimation in vectored systems the “sequence” formed byrandom data sent by a CPE device coupled to a legacy line is not apriori known to the CO device, the “sequence” as received on the legacyline may be taken as a basis for estimating crosstalk from the legacyline to the vectored line (essentially by similar algorithms as used inconventional vectoring, with the exception that instead of onlypredetermined orthogonal test sequences on one or more legacy linesrandom data or other statistically random signals transmitted by alegacy CPE device are used in embodiments). Based on error values forthe vectored lines (where predetermined sequences are used and thuserror values may be determined), crosstalk between the vectored linesand from the legacy lines to vectored lines may be determined.

Similar as discussed for the downstream case, crosstalk estimatesobtained in this way may be re-used for a next training of a respectivelegacy line, and/or for example also as a starting point for crosstalkestimation for a next training.

In some embodiments, also a downstream crosstalk transfer functionpreviously estimated, for example as described above, may be used as astarting point for estimation of crosstalk transfer in the upstreamdirection. This makes use of the fact that in some systems crosstalk inthe upstream direction and in the downstream direction may at least tosome extent behave in a similar manner.

Depending on the estimated crosstalk coefficients for the downstreamand/or upstream direction, in some embodiments also parameters forSeamless Rate Adaptation (SRA) or Save Our Showtime (SOS) may beadapted.

In some embodiments, by a small firmware update of legacy CPE devicesthe CPE devices may also be instructed to transmit only synchronizationsymbols at the beginning of initialisation.

The above-described embodiments serve only as examples and are not to beconstrued as limiting.

What is claimed is:
 1. A communication device comprising: a plurality offirst transceivers configured to communicate with a respective pluralityof second transceivers over a plurality of communication lines; avectoring device coupled to the plurality of first transceivers; atleast one of the first transceivers being configured to supportcommunication with a legacy transceiver of the plurality of secondtransceivers in the upstream direction, wherein the legacy transceivercomprises a transceiver that is not adapted to vectoring; thecommunication device being adapted to estimate crosstalk from a legacyline of the plurality of communication lines coupled to the legacytransceiver, to at least one vectored line of the plurality ofcommunication lines coupled to a vectored transceiver of the pluralityof second transceivers, based on the said communication with the legacytransceiver in the upstream direction.
 2. The communication device ofclaim 1, wherein a transmit power of the legacy transceiver is reduceduntil crosstalk coefficients have been adapted.
 3. The communicationdevice of claim 1, wherein during a crosstalk adaptation time a marginof the vectored transceiver is increased.
 4. The communication device ofclaim 1, wherein the communication with the legacy transceiver is duringshowtime of the legacy transceiver.
 5. The communication device of claim2, wherein during operation with reduced transmit power, crosstalk fromthe legacy line to vectored lines is determined, and vectoring isadapted correspondingly.
 6. The communication device of claim 1, whereintwo trainings of the legacy transceiver is implemented.
 7. Thecommunication device of claim 6, wherein for a first training a reducedupstream transmit power is demanded from the legacy transceiver by thecommunication device to minimize the crosstalk into the vectored lineuntil crosstalk coefficients are adapted; and wherein for a secondtraining, executed after crosstalk adaptation, a full upstream transmitpower is applied, to train the legacy line for transmission with fulltransmit power, wherein the full transmit power comprises a maximumtransmit power supported on the legacy line.
 8. The communication deviceof claim 1, wherein the vectored line is protected in showtime byreducing its upstream bit loading when the legacy line joins.
 9. Thecommunication device of claim 8, wherein reduction of upstream bitloading is achieved by forcing an upstream Seamless Rate Adaptation(SRA) or a Save Our Showtime (SOS) by the communication device.
 10. Thecommunication device of claim 1, wherein data symbols transmitted inshowtime are used for estimating upstream crosstalk from the legacy lineto the vectored line.